AltiVec is a floating point and integer SIMD instruction set designed and owned by Apple, IBM and Freescale Semiconductor, formerly the Semiconductor Products Sector of Motorola, (the AIM alliance), and implemented on versions of the PowerPC including Motorola's G4, IBM's G5 and POWER6 processors, and P.A. Semi's PWRficient PA6T. AltiVec is a trademark owned solely by Freescale, so the system is also referred to as Velocity Engine by Apple and VMX by IBM and P.A. Semi, although IBM has recently begun using AltiVec as well.

While AltiVec refers to an instruction set, the implementations in CPUs produced by IBM and Motorola are separate in terms of logic design. To date, no IBM core has included an AltiVec logic design licensed from Motorola or vice-versa.

AltiVec is a standard part of the new Power ISA v.2.03[1] specification. It was never formally a part of the PowerPC architecture until this specification although it used PowerPC instruction formats and syntax and occupied the opcode space expressly allocated for such purposes.